What is allergic contact dermatitis?
Allergic contact dermatitis or contact allergy arises when the skin comes in contact with irritating substances or allergens that trigger an allergic reaction. Allergen is classified as anything that causes the skin to become red, itchy, swollen and blistered or dry. These substances are individual-specific and cause no reaction in those that are not sensitive to it. Additionally, the allergens can result in worsening of eczema symptoms. It is necessary that you are aware of the substances or chemicals to which the skin is sensitive.
According to the Mayo Clinic, the most recent list of top ten contact allergens include:
- Nickel (nickel sulfate hexahydrate) — metal frequently encountered in jewellery and clasps or buttons on clothing
- Gold (gold sodium thiosulfate) — precious metal often found in jewellery
- Balsam of Peru (myroxylon pereirae) — a fragrance used in perfumes and skin lotions, derived from tree resin
- Thimerosal — a mercury compound used in local antiseptics and in vaccines
- Neomycin sulfate — a topical antibiotic common in first aid creams and ointments, also found occasionally in cosmetics, deodorant, soap and pet food
- Fragrance mix — a group of the eight most common fragrance allergens found in foods, cosmetic products, insecticides, antiseptics, soaps, perfumes and dental products
- Formaldehyde — a preservative with multiple uses, e.g., in paper products, paints, medications, household cleaners, cosmetic products and fabric finishes
- Cobalt chloride — metal found in medical products; hair dye; antiperspirant; objects plated in metal such as snaps, buttons or tools; and in cobalt blue pigment
- Bacitracin — a topical antibiotic
- Quaternium 15 — preservative found in cosmetic products such as self-tanners, shampoo, nail polish and sunscreen or in industrial products such as polishes, paints and waxes
The list above is retrieved from https://www.sciencedaily.com/releases/2006/03/060303204044.htm
Diagnosis of Contact Allergy
To avoid eczema flare-ups and allergic reactions, the allergen needs to be identified. It is easy to diagnose the causative allergen without performing any specific tests. Your doctor might be able to analyze the severity of contact dermatitis and identify its cause simply by:
- Obtaining details regarding your signs and symptoms
- Inquiring about suspected substances that may trigger allergic reactions
- Examining your skin to identify the pattern and acuteness of the rash
In some cases, doctors may carry out open application test to detect if the causes contact allergy is due to a cosmetic product like moisturizer. You will be asked to apply the suspected product many times daily for several days to a patch of sensitive skin. If, the treated patch shows specific symptoms, it is likely to be contact allergy.
However, in severe cases, the dermatologist might recommend certain skin tests to determine the cause behind the symptoms. Patch test is more commonly suggested by most of the doctors to detect the allergen and the symptoms of allergic contact dermatitis.
What is patch testing?
Patch testing is a process to identify allergic contact dermatitis to something a person has contacted at home, leisure or at work. It involves applying patches with test material in small hall or discs to a person’s back. The patches are secured with hypoallergenic tapes. No needle pricking is involved. Patch testing does not test for urticarial (hives) or food allergy.
Patch testing for contact dermatitis
Your dermatologist may recommend patch test to check if your skin is allergic to a particular substance or chemical and resulting in allergic skin irritation. This test is more useful in cases when the cause of the symptoms isn’t apparent or if the skin rash often recurs. Also, patch tests are often used to detect allergic reactions that otherwise take several days to develop.
How is patch testing done?
- During a patch test, the skin is exposed to small amounts of susceptible substances that can cause contact allergy. These substances may include latex, medications, fragrances, preservatives, hair dyes, metals and resins.The patches are removed after 48 hours and the skin is tested for allergy.
- During the first visit for patch test: the doctor will apply allergen-treated patches to your skin. More commonly the patches are applied on the back or arm. The patches are secured using hypoallergenic adhesive tape and kept undisturbed for the next two or three days. The doctor will strictly recommend you to avoid bathing or involving in activities that cause heavy sweating.
- During the second visit (usually after 48 hours): the patches will be removed and the test sites will be marked using a suitable marker to keep a track. At times, additional patches may be applied, if required.
- During the third visit (usually after 24-48 hours): the test sites are examined to detect irritated skin or rashes.
- Depending upon the skin reactions, the doctor will decide the course of treatment and whether further testing is required.
Patch Testing and Skin Prick Test
There is a difference between skin prick test and patch test. While skin prick test is often used to detect immediate allergic reactions in patients, patch test is deployed to detect delayed allergic reactions that take days to develop.
Skin Prick test: Prick testing or scratch test is usually carried out to identify allergies arising because of pollen, mold, pet dander, dust mites and foods. It tests type I allergy that causes hay fever, asthma and contact urticaria (hives). Needles are used for this test and readings are taken 15-20 minutes after the pricks.
Patch test: Patch testing is used to detect type IV hypersensitivity reaction. This test mainly includes testing of chemicals like hair dye, latex, preservative and fragrances in sunscreens, cosmetics and medicaments. The reactions may show in 2 to 4 days after application of the allergen on the skin but in few cases, it can take up to 7 days to react.